This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. ABSTRACT: Secretory IgA (SIgA) antibodies coat the mucosal surfaces of the gastrointestinal tract and serve as the first line of defense against Gram negative enteric pathogens, including Escherichia coli, Shigella flexneri, Vibrio cholerae, and Salmonella typhimurium. It is generally assumed that SIgA protects mucosal surfaces primarily by binding bacterial lipopolysaccharide and promoting bacterial agglutination, thereby, preventing bacterial contact with epithelial surfaces. However, recent studies from our laboratory in collaboration with Drs. William Samsonoff and Richard Cole suggest that SIgA antibodies directly interfere with bacterial pathogenesis by promoting changes in the bacterial outer membrane that arrest flagella rotation and block secretion through specialized secretion apparatuses. Specifically, we propose that SIgA against LPS causes localized distortions of the outer membrane that result in the opening of mechanosensitive pores and the subsequent dissipation of the proton motive force, which is know to drive both flagella rotation and secretion. We propose to use cryoelectron tomography to visualize the outer membrane of Salmonella typhimurium in the absence and presence of the anti-LPS monoclonal IgA antibody Sal4. In preliminary studies, we have optimized antibody and bacteria concentrations required to arrest bacterial motility and invasion of epithelial cells. Based on work presented by Dr. Manella, we expect that cryoelectron tomography will enable us to visualize in great detail the bacterial flagella, secretion apparatuses and outer membrane architecture.